coallocate socket_wq with socket itself

socket->wq is assign-once, set when we are initializing both
struct socket it's in and struct socket_wq it points to.  As the
matter of fact, the only reason for separate allocation was the
ability to RCU-delay freeing of socket_wq.  RCU-delaying the
freeing of socket itself gets rid of that need, so we can just
fold struct socket_wq into the end of struct socket and simplify
the life both for sock_alloc_inode() (one allocation instead of
two) and for tun/tap oddballs, where we used to embed struct socket
and struct socket_wq into the same structure (now - embedding just
the struct socket).

Note that reference to struct socket_wq in struct sock does remain
a reference - that's unchanged.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/net.h

@@ -116,11 +116,11 @@ struct socket {
 
 	unsigned long		flags;
 
-	struct socket_wq	*wq;
-
 	struct file		*file;
 	struct sock		*sk;
 	const struct proto_ops	*ops;
+
+	struct socket_wq	wq;
 };
 
 struct vm_area_struct;